Rapid Shift

Ag + solar solutions in fields and smart solar greenhouse

Putting solar panels on a roof is a good idea. Growing food in a greenhouse is also a good idea. But putting those ideas together usually doesn’t work because the solar panels block out the sunlight the plants inside need to flourish. Researchers at the University of California, Santa Cruz, have created new solar panel technology they call Wavelength Selective Photovoltaic Systems. It uses part of the visible light spectrum to generate electricity while letting the rest of the light through to the plants growing inside. The results have been encouraging.

Wavelength Selective Photovoltaic Systems (WSPV) uses transparent roof panels embedded with a magenta luminescent dye to absorb some of the blue and green wavelengths of sunlight and transfer it to narrow photovoltaic strips that generate electricity. The other wavelengths pass through to the interior of the greenhouse. The result? Solar power that costs less per watt than conventional rooftop systems, and an increase in growth for many of the fruits and vegetables inside.

Inside two experimental greenhouses — one on campus and the other in Watsonville, California — the team raised 20 varieties of tomatoes, cucumbers, lemons, limes, peppers, strawberries, and basil. “80% of the plants weren’t affected, while 20% actually grew better under the magenta windows,” reports Michael Loik, professor of environmental studies at the University of California, Santa Cruz.

He is the lead author of a paper about the program published in the current issue of the American Geophysical Union’s journal Earth’s Future. “We have demonstrated that ‘smart greenhouses’ can capture solar energy for electricity without reducing plant growth, which is pretty exciting.” There’s more good news. The plants in the WSPV-equipped greenhouses used 5% less water than those raised in conventional facilities.

“I thought the plants would grow more slowly, because it’s darker under these pink panels. The color of the light makes it like being on the Red Planet,” says Loik. “Plants are sensitive not just to the intensity of light but also to color. But it turns out the plants grow just as well.”

Using greenhouses to grow fruits and vegetables around the world has increased 600% over the past 20 years, and greenhouses now cover the equivalent of 9 million acres — twice the size of New Jersey. “It’s big and getting bigger,” Loik says. “Canada relies heavily on greenhouses for vegetable production and their use is growing in China, too.” Tomatoes and cucumbers are among the top greenhouse-produced crops worldwide. Plastic greenhouses are also becoming popular for small-scale commercial farming and household food production, he adds.

Greenhouses consume a lot of electricity to run the fans, lights, pipes, and monitoring systems inside. “This technology has the potential to take greenhouses offline,” says Loik, who specializes in climate change, plant physiology, water resources, and sustainable technologies. The cost per watt of the WSPV system is 65 cents per watt — about 40% less than the per-watt cost of traditional silicon-based photovoltaic cells. “If greenhouses generate electricity on site, that reduces the need for an outside source, which helps lower greenhouse gas emissions even more,” he adds. “We’re moving toward self-sustaining greenhouses.”

This is yet more good news about the marriage of solar power and agriculture. The Fraunhofer Institute For Solar Energy Systems in Germany is reporting that solar panels mounted high above farmland can increase the efficiency of each acre of land by as much as 60%. Global warming is already changing traditional farming. Greenhouses may be vital to feeding the planet in years to come as climate change makes some agricultural areas less productive, reducing yields per acre.

Out in the fields…

Solar panels are wonderful things, but they do take up a lot of space, especially for larger, utility-scale systems. In some densely populated countries like China and India, where loss of farmland can lead to hungry people, floating solar farms are being built to take advantage of the surface area of lakes and rivers. Researchers at the Fraunhofer Institute For Solar Energy Systems have conducted an experiment near Lake Constance — which borders Germany, Lichtenstein, and Switzerland — regarding another solution.

According to a Fraunhofer press release, the experiment involves 720 bi-facial solar panels covering about a third of a hectare of agricultural land (on the Demeter farm cooperative Heggelbach). The panels are mounted high enough to allow the crops planted below to receive almost as much sunshine as they would if the panels were not there and to permit farm machinery to operate beneath them. After a year of trials, the research showed the dual use system increased the total productivity of the land by 60%.

Fraunhofer refers to the dual use system as “agrophotovoltaics,” or APV. “APV has the potential to open up new space that is urgently needed for the PV expansion in Germany, says professor Hans-Martin Henning, the director of Fraunhofer ISE. “At the same time, APV can mitigate the conflicting interests between agriculture and open space PV systems for viable land. Before market readiness, however, other sectors and differently sized systems still must be tested. Also, the technical integration must be further advanced, for example, the implementation of storage.”

The first crops tested were winter wheat, potatoes, celeriac, and clover grass. “The crop yield of clover grass under the PV array was only 5.3 percent less than the reference plot,” reports professor Petera Högy, an agricultural expert at the University of Hohenheim. The yield for potatoes, wheat and celeriac were about 19 percent less.

“From the perspective of agricultural science, agrophotovoltaics is a promising solution for increasing both the land use efficiency and the share of renewable energy provided by the agricultural sector,” says professor Iris Lewandowski, who heads up the department of biobased products and energy crops at the University of Hohenheim.

The 194 kilowatt solar installation generated 1266 kilowatt-hours of electricity per installed kilowatt, one third more than the average value of 950 kilowatt-hours per kilowatt in Germany. 40% of the power produced was used to charge the batteries of the electric farm equipment and harvest crops. The team believes 70% of the energy could be utilized if a storage battery was included in the system. Any excess electricity was sold to Elektrizitätswerke Schönau, an electric utility company that uses 100% renewable energy and is a partner in the project.

“In order to provide the necessary proof-of-concept before market entry, we need to compare further techno-economical applications of APV, demonstrate the transferability to other regional areas and also realize larger systems,” says project manager Stephan Schindele. Experiments involving solar in combination with fruits, berries, hops, and grapes are planned for the future, along with various technologies such as energy storage, special films with organic solar cells, and solar PV water treatment systems.

While more research is needed, the initial results indicate that APV is a significant step forward for solar power in agricultural settings.

Steve Hanley writes about the interface between technology and sustainability from his home in Rhode Island. You can follow him on Google + and on Twitter. “There may be times when we are powerless to prevent injustice, but there must never be a time when we fail to protest.” Elie Wiesel